The Martian atmosphere’s not what it used to be. Solar winds bombard the planet, taking gas molecules (represented by colored streaks in the image above) with them. New measurements of atmospheric loss by NASA’s MAVEN probe should help scientists determine how a planet with rushing water and a temperate climate a few billion years ago transformed into a cold, dry desert.

Atmospheric “loss to space was a significant, if not dominant, process in changing the climate,” says MAVEN principal investigator Bruce Jakosky of the University of Colorado Boulder.

The key factor in the atmospheric demise is that unlike Earth, Mars doesn’t have a global magnetic field (see illustrations below). As a result, the planet can’t protect itself from particles and plasma streaming from the sun. Mars loses about 100 grams of its atmosphere every second, MAVEN researchers report in the Nov. 6 Science.

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